Improved how staircases are drawn in 3d maze. Added option to have arbitrary outdoor background shown through a type 4 door.
MazeScreen wall/floor/line/stairs colors are now settable per map by putting a colorPalette property on the map object.
Issue #68 - refactored the way we create 3d geometry so rotating it any direction is easier. Fixed the bounding boxes around staircases. Eliminated the bgFaces array -- all polygon faces now obey the same clipping rules.